In a nuclear power plant, the fission process produces radioactive iodine, along with various other radioactive and non-radioactive substances. During a nuclear accident, radioactive iodine may be released. Radioactive iodine possesses the same general characteristics as natural iodine: like natural iodine, radioactive iodine enters the body and accumulates in the thyroid gland. This has one major disadvantage: this uptake of radioactive iodine may increase the risk of thyroid cancer, especially in children and young people.

However, if tablets containing a high concentration of non-radioactive iodine are taken in time, the thyroid gland will be saturated with "healthy" iodine so that it cannot absorb any radioactive iodine. This procedure, known as thyroid blocking, thus prevents the radioactive iodine from accumulating in the thyroid. The important factor is to ensure that the iodine tablets are taken at the right time to maximise their effectiveness.